The "Instant" SSID is the provisioning SSID so you can configure the AP over the air. This disappears after your real SSIDs are configured and someone has successfully attached to the real SSIDs. Also, if one AP has been configured it becomes the "master" AP for the group -- we call it the "Virtual Controller (VC)". When a new member AP joins the group the VC automatically pushes the config to the new member so you will never see the "Instant" SSID from the new AP.
Auto-Join is enabled by default and it allows new member APs to join the group.
It is possible that your third AP is unable to join due to a SW mismatch. Check the version on the two APs then check the version on the third.
Also, make sure that "Extended SSID" support is disabled as this would prevent the mesh link from coming up.